v TEETH OF PLATYPUS 113
teen pairs of ribs and only two lumbar vertebrae. The skull is
expanded in front, and the bill is supported by two, at first
diverging, and then converging, premaxillae. Between them is
the famous " dumb-bell shaped bone/' which is believed to be the
representative of the reptilian prevomer. The pterygoids are
smaller than in JEcliidna,, and the hard palate does not extend
so far back as in that genus. The brain of this genus is
The discovery of the real teeth of OrnithorJiynchus only dates
from the year 1888, when they were found by Professor Poulton *
in an embryo. Later Mr. Thomas found 2 that the teeth persist
FIG. 55.—Duck-billed Platypus. Ornithor7iync7ius anatimis. x %.
for a considerable portion of the animal's life, and are only shed,
like milk teeth, " after being worn down by friction with food
and sand." "We have already (p. 98) called attention to the
general similarity of these teeth to those of certain of the
earliest Mammalia and of mammal-like reptiles. The teeth are
all molars, and they are either eight or ten in number. They
are replaced by the horny plates of the adult animal; but the
mode of replacement is curious. The plates are developed from,
the epithelium of the mouth, but round and under the true
teeth ; the epithelium, of the mouth grows gradually under the
calcified teeth, a method of growth which has possibly some-
thing to do with the shedding of the latter. The hollows and
1 Quart. J. Micr. Set. xxix. ISSl ±>. «*>*.
2 l>t'oc. llvy. Soc. xlvi. 1889, p. 127. Sec also Stewart, Quart. J. Micr. Sci. x>*xiii.
1892, p. 229.